In general, when an internal combustion engine such as a diesel engine (hereinafter, referred to as an “engine”) is in operation, all of energy generated by combustion of mixture gas cannot be transduced to a work to rotate a crankshaft, and thus, a loss is inevitably generated. The loss includes a cooling loss transduced to an increase in temperature of the engine itself and cooling water, an exhaust loss emitted to the atmosphere by exhausted gas, a pumping loss generated as intake and exhaust, a mechanical resistance loss, and so on. Among these, the cooling loss and the exhaust loss account for a significant proportion of the entire loss. Thus, in order to improve fuel consumption ratio, it is effective to reduce the cooling loss and the exhaust loss.
However, in general, the cooling loss and the exhaust loss are in trade-off relationship, and therefore, it is difficult to reduce both of the cooling loss and the exhaust loss at the same time in most cases. For example, in a case in which an engine has a supercharger, the exhaust loss decreases as the supercharging pressure increases, since the energy of the exhausted gas is effectively utilized. On the other hand, combustion temperature increases as the compression ratio substantially increases, and thus, the cooling loss increases. Accordingly, in some cases, the sum of those losses may increase.
In order to reduce the sum of the losses, a control apparatus which controls a combustion state of fuel supplied (injected) to an engine (hereinafter, simply referred to as a “combustion state of an engine”) needs to control optimally various parameters which change the combustion state such as a fuel injection amount and an injection timing, and an EGR gas amount, in addition to the supercharging pressure described above, in response to an operational state of the engine (a rotational speed, an output power, and so on). The parameters which change the combustion state (that is, the parameters which affect the combustion state described above) are simply referred to as “combustion parameters”. It is difficult, however, to determine the combustion parameters through experiments or the like in advance such that those coincide with optimal values in regard to each operational state, and thus, it is necessary to conduct an enormous number of experiments in order to determine those. Therefore, methods to determine the combustion parameters in a systematic manner have been developed.
For example, one of conventional control apparatuses for an internal combustion engine (hereinafter, referred to as a “conventional apparatus”) calculates a crank angle at which a half of total heat amount generated during a combustion stroke is generated (hereinafter, the crank angle being referred to as a “Combustion Barycentric Angle”). Furthermore, when the Combustion Barycentric Angle is different/depart from a predetermined reference value, the conventional apparatus makes the Combustion Barycentric Angle become equate to the reference value by correcting injection timing or by adjusting oxygen density in combustion chambers (in cylinders) through adjustment of an EGR ratio (an amount of EGR gas) (e.g., refer to Patent Literature 1).